MIAMI — A dramatic change in the Gulf of Mexico's loop current has trapped a slick of oil in a huge circular eddy that scientists said Thursday appears likely to push slowly west instead of pumping the oil south into the Florida Keys and possibly up the East Coast of the United States.
The shift, which oceanographers have been watching strengthen for a week, has at the least reduced the imminent environmental threat for Florida. Tar balls predicted to be floating in the Florida Straits by now instead might not arrive for weeks, months or — depending on lots of variables — maybe at all.
"I don't think there is an express lane here any more," said Nick Shay, a physical oceanographer at the University of Miami Rosenstiel School of Marine and Atmospheric Science.
But he and other scientists caution they are still struggling to assess another unseen but potentially catastrophic threat — plumes of submerged oil from the April 20 deep sea blowout that the federal government finally confirmed has become the nation's largest oil spill.
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A team of scientists assembled by the federal government calculated the flow at two to five times the rate estimated by BP — meaning from 18 to nearly 40 million gallons of crude have already spewed into the Gulf. That's roughly two to four times the 11 million gallons the Exxon Valdez dumped in Alaska in 1989.
There is mounting evidence a considerable amount remains below the surface.
How much is there and where it is going is both a major concern and mystery, said Frank Muller-Karger, biological oceanographer at the University of South Florida.
"We need to know what is down below," said Muller-Karger. "We have no handle on that at all."
A team from a USF research vessel reported Thursday that it had mapped a massive undersea plume of hydrocarbons, a strong indicator of oil — stretching 22 miles north of the well, six miles wide and starting just below the surface down to 3,300 feet, with the heaviest concentrations at 1,300 feet.
It was the second deep-sea cloud discovered by researchers surveying the spill. Samantha Joye, a University of Georgia marine scientist leading a team that first discovered large underwater plumes two weeks ago, also reported Thursday on her shipboard blog that a second voyage had found a plume moving north at 2,500 to nearly 4,000 feet.
And Shay, who is partnering with the National Oceanic and Atmospheric Administration on reconnaissance flights, believes instruments he's dropping to record ocean temperature and salinity have pinpointed yet more plumes drifting from 160 to nearly 500 feet deep.
If preliminary readings are verified, impacts from the spill could rise exponentially — potentially contaminating marine life from the deep sea floor to the shoreline. Submerged oil could wreak havoc on deep ocean creatures, from filter feeders like whales to plankton and larvae near the surface.
"The first ecological impact of this spill is the effect on coastal habitats, including marshes, beaches and estuaries. The second threat to nature would be the impact on the food webs," David Hollander, a chemical oceanographer who was lead investigator for the USF voyage, said in a release. "That is what's at risk."
Scientists believe temperature and salinity gradients could be trapping the oil as well as other possible factors, including the deep sea application of chemical dispersants, which break the oil into clouds of tiny droplets so diffused they might rise very slowly or even remain trapped by layers of warmer, heavier water overhead.
Shay speculated that plumes might take months to surface, if they surface at all.
"It doesn't surprise me we haven't seen any of these blobs pop to the surface in the loop current," he said.
Tracking and estimating the size of sub-sea plumes pose huge technical challenges. Unlike surface slicks, there is no satellite image to track and Coast Guard studies as recent as last year call the technology to assess submerged oil inadequate.
The uncertainty of where the oil might be going adds to the concern. The USF surveys suggest oil is moving north up the Continental Shelf toward Louisiana. But Shay's readings came south of the spill site, raising concerns that unseen oil could still make its way toward the Florida coast.
For now, what oceanographers can clearly track is the change in the loop current, a warm water pipeline constantly reshaping itself. Two weeks ago, Muller-Karger said, it looked much like a horseshoe, digging far into the Northern Gulf near the spill.
But over the last two weeks, a counter-clockwise eddy on the outside of the loop began pushing east may force the current into a circular pattern oceanographers call a "warm core eddy."
"In oceanography, that is not a new phenomenon," Muller-Karger said. "The horseshoe closes up on itself and you have this big ring of water the size of Florida."
That ring, now about 100 miles west of Tampa, typically will drift west — bad news for Texas and Mexico but a good development for the Florida Keys. The remaining loop current typically flattens out, flowing east across the Yucatan Channel to the Florida Straits — hundreds of miles south of the spill — before moving up the East Coast.
It's possibly the formation can change, scientists say, and eddies can reattach but the process can take weeks or months, essentially confining the surface slick already drawn into the loop in the mid-Gulf for the near future.
Scientists say it is much harder to predict what might happen to submerged oil if it moves south. It's possible, for instance, that seasonal upwelling of cold water in the summer could push it out below the eddy and loop current onto the shallow Continental Shelf west of Florida, Muller-Karger said.
If that happened, winds and weather would take over.
"After it's on the shelf, it can affect anywhere along the coast," he said.